1. Field of the Invention
This invention relates generally to the transfer of epoxy, or other molding plastics or materials, into mold cavities for encapsulation of devices within the mold cavity, and more specifically, to semiconductor device encapsulating equipment wherein an epoxy transfer plunger has a second plunger placed inside the epoxy transfer plunger which serves both as an epoxy quantity compensatory and as an epoxy curing-pressure transmission device, and method therefor.
2. Description of the Related Art
In a transfer molding process, such as is used for semiconductor device packaging, a plunger transfers pelletized epoxy through a canal, or runner, through a cavity opening, or gate, and into a cavity. This transfer molding process may also utilize a plurality of cavities each having its own canals, gates, etc. After a cavity is filled with the epoxy which covers the semiconductor device located therein, usually mounted on a leadframe, the pressure applied to the liquid epoxy is increased by an application of force applied by the plunger. This is done in order to achieve a desired epoxy density and also to ensure that any air in the epoxy is removed during the curing of the epoxy into a solid form.
Among the methods used for the transfer molding process are single-plunger molding, or conventional molding, in which an epoxy fill of all cavities is performed by one plunger; and multi-plunger molding in which an epoxy fill of the cavities is performed by more than one plunger. Thus, though the following discussing addresses multi-plunger molding, the discussion is also applicable to single-plunger molding and other methods known to those skilled in the art. The plungers can be driven by a variety of means of applying motive force including: hydraulic, pneumatic, mechanical, electrical or combinations thereof.
Many prior art methods of transfer molding processes used hydraulics, acting to drive the plunger(s), as the transfer motive force due to the ability to easily accommodate variable pressure controls in order to provide epoxy quantity compensation. However, more recently, the semiconductor industry has been turning away from the use of hydraulics because of the production of oil vapors from the hydraulic fluids and the associated contamination issues.
Presently, electro-mechanical based transfer drives incorporating compression springs are preferred and in general use. Typically, such a transfer drive utilizes an electro-mechanical transfer drive to supply the motive force to each and all the plungers, and each individual plunger has a pre-tensioned compression spring.
Several reasons favor such an arrangement: The epoxy pellets are subject to weight variation or in-tolerances either as supplied or due to damage. A lighter weight pellet has less epoxy material which is reflected in a lower volumetric displacement. Thus, the smaller amount of epoxy material requires adjusting the stroke of the plunger, or plungers, acting on the epoxy pellet to compensate for this reduced displacement. This displacement compensation, or distance dissipation, requirement is satisfied through the use of a compression spring coupled to each individual plunger thus enabling a single transfer drive to be used for supplying the motive force to a plurality of plungers pushing them all to an end position where each compression spring adapts, or compensates for, the precise epoxy quantity present for that specific plunger.
Additionally, in the event that any epoxy residue from a previous molding cycle used for semiconductor device encapsulation is left in the runner, or gate, blocking off the epoxy passage, a condition called gate-lock, the compression spring can compensate for the blocked runner or gate while allowing the other plungers to continue with and complete the curing phase.
Furthermore, following completion of the transfer molding process, the motive force on the plungers must be increased in order to facilitate the application of curing pressure to the epoxy located in each cavity covering its associated semiconductor device. The transfer drive compresses the springs and pushes the plungers with a higher force, which results in an increased pressure in the cavities during the epoxy curing stage.
However, the use of compression springs also has some disadvantages. For example, (1) in the event that epoxy contamination is present in the plunger cylinder, the plunger's movement will be slowed or arrested until the force built up in the compressed spring overcomes the resistance from the epoxy contamination. When the resistance is overcome, the plunger will resume movement, but now the force stored in the compressed spring will be released suddenly and added to the drive force at the input to the system. This results in a sudden acceleration of the plunger which translates into a sudden increase in the epoxy injection flowrate. Furthermore, successive areas of epoxy contamination will cause this unpredictable plunger movement to be repeated. The results of uneven and excessive epoxy flowrates can be a phenomenon known as wire sweep. Wire sweep is the breaking or bending of the gold wires connected between the semiconductor chip and leads or pins of the leadframe. Additionally, as epoxy contamination is neither predictable nor controllable this renders the transfer molding process unpredictable and non-uniform. (2) An additional disadvantage is that the displacement compensation results in a varied and unpredictable pressure at the start, during, and end of the curing phase thus resulting in varied and unpredictable curing process pressures both individually, and from cavity to cavity. (3) Yet another disadvantage is that in the event of a gate-lock, the drive has to continue its movement and compresses the compression spring over a relatively large distance which results in a relatively great force on the drive and a high pressure in the present runner while a fault situation is present.
Because of the above disadvantages, the selection of a compression spring is a compromise between addressing these three disadvantage issues and their contrary compression spring demands. The compromise is in the selection of the spring constant, the spring length and the spring pre-tension. For example, a high compression spring pre-tension is beneficial in addressing issue (1), but a high compression spring pre-tension also increases the minimum possible process pressure which may affect the curing process. In addressing the displacement compensation of issue (2), the typical spring compression distance will be small. In contrast however, the spring compression distance in the event of a gate-lock, as in issue (3), will require a much longer spring compression distance. But again, this is a compromise because a low spring constant and a long length compression spring complicates the guiding and movement of the plunger and also results in a relative long transfer drive.
Therefore a need existed for a transfer molding system that does not utilize compression springs in order to eliminate the erratic epoxy flowrates caused by epoxy contamination in combination with compression springs. Another need existed for a transfer molding system with reduced susceptibility to wire sweep. A further need existed for a transfer molding system having essentially controllable and repeatable flow and pressure values for the transfer molding process. Yet a further need existed for a transfer molding system that would not be subject to the mechanical stresses from gate-lock inherent in a transfer molding system having compression springs.